1. Field of the Invention
The present invention relates to a mobile radio communication system, and more particularly to a digital mobile radio communication system having a service area composed of a plurality of radio zones for automobile telephone terminals.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows in block form a general mobile radio communication system for use in automobile radio communications or the like.
The mobile radio communication system shown in FIG. 1 comprises a plurality of mobile stations 100-1 .about.100-j, a plurality of radio base stations 101-1, 101-2 for effecting radio communication with the mobile stations 100-1 .about.100-j through radio communication paths or circuits using radio frequencies, and a host switching office 104 composed of a base station controller 102 and an exchange apparatus 103. The base station controller 102 controls the radio base stations 101-1, 101-2, and connects calls to and from the mobile stations 100-1 .about.100-j through the radio base stations 101-1, 101-2. The exchange apparatus 103 selects communication lines with switching circuits to send communication signals supplied from the mobile stations 100-1 .about.100-j through the radio base stations 101-1, 101-2 and the base station controller 102 to a public communication network, and also to send communication signals supplied from the public communication network through the base station controller 102 and the radio base stations 101-1, 101-2 to the mobile stations 100-1 .about.100-j.
The radio base station 101, identical to each of the radio base stations 101-1, 101-2, and the base station controller 102 are shown in block form in FIG. 2 of the accompanying drawings. FIG. 3 of the accompanying drawings illustrates signals which are demultiplexed from a multiplex signal by a multiplexer/demultiplexer 9 (see FIG. 2) in the base station controller 102.
As shown in FIG. 2, multiplexer/demultiplexer 9 is supplied with a multiplex signal that is transmitted from the exchange apparatus 103 at a transmission rate of 2 Mbps, and demultiplexes the supplied multiplex signal into signals S1-1, S1-2, . . . , S1-k at 64 Kbps for use in a time-division switching operation. As shown in FIG. 3, these demultiplexed signals 22 are time-division signals composed of 32 time slots TS0 .about.TS31. Each of the time slots TS0 .about.TS31 has auxiliary time slots TS0(R), TS1(R), TS2(R) (R indicates RADIO time slot) for accommodating audio/data signals for mobile radio communication in four channels or control signals for controlling the connection of calls. The base station controller 102 also has a switch circuit 7 which is a time-division switching switch for separating the demultiplexed signals for the radio base station 101-1 or 101-2 and transmitter/receivers capable of communicating with corresponding mobile stations covered by the radio base station.
A control signal processor 11 sends control signals to and receives control signals from the mobile stations through the radio base station 101 to control switch circuit 7. The base station controller 102 further includes a first group of CODECs (coder-decoders) 12-11, 12-12, 12-13 corresponding to a transmitter/receiver 3-1 in the radio base station 101, a second group of CODECs 12-21, 12-22, 12-23 corresponding to a transmitter/receiver 3-2 in the radio base station 101, and an nth group of CODECs 12-n1, 12-n2, 12-n3 corresponding to a transmitter/receiver 3-n in the radio base station 101. Each of the groups of CODECs serves to convert distributed signals of 64 Kbps outputted from switch circuit 7 into signals a1, a2, . . . , an of 16 Kbps to be distributed for radio communication in the mobile radio communication system. The number k of the demultiplexed signals S1-1, S1-2, . . . , S1-k outputted from multiplexer/demultiplexer 9 differs from the number n of the CODEC groups because not all of the 32 time slots of the multiplex signal of 2 Mbps are used, and the number n of the CODEC groups may be greater or smaller than the number of the time slots depending on the system configuration.
As shown in FIG. 3, each of the signals a1, a2, . . . , an to be distributed for radio communication has auxiliary time slots TS0(R) .about.TS3(R) for four channels. Usually, the auxiliary time slots TS0(R) .about.TS2(R) are assigned audio/data signals to be transmitted to the mobile station, and the auxiliary time slot TS3(R) is assigned a monitor signal for rate conversion. A call connecting signal a0 is not converted with respect to the transmission rate, but inserted in a time slot for the mobile station to which a call is to be connected, e.g., the auxiliary time slot TS0(R) if it is an empty slot, in the corresponding transmitter/receiver in the radio base station 101 to which it is supplied through a multiplexer/demultiplexer 6 in the base station controller 102. The multiplexer/demultiplexer 6 multiplexes the output signals a1, a2, . . . , an from the CODEC groups and the call connecting signal a0, and transmits a multiplex signal to the radio base station 101.
In FIG. 2, only one output signal line is shown as extending from multiplexer/demultiplexer 6. Actually, however, multiplexer/demultiplexer 6 has as many output signal lines as the number of the radio base stations. Switch circuit 7 determines which radio base station the multiplex signal is to be transmitted to, under the control of a control unit 13 which is capable of detecting which mobile station the radio base station 101 is communicating with and also of determining which radio base station the signals of 16 Kbps generated by the CODECs are to be transmitted to.
The base station controller 102 is geographically spaced from the radio base stations 101-1 .about.101-2 by several tens of kilometers and communicates with the radio base stations which cover a number of indefinite mobile stations. Therefore, the mobile radio communication system employs a serial signal of 2 Mbps. Though the mobile radio communication system is assumed to employ a serial signal of 2 Mbps for transmission between the exchange apparatus 103 and the base station controller 102 or between the base station controller 102 and the radio base stations 101-1, 101-2, it may employ a serial signal of 1.5 Mbps or a signal of 8 Mbps or 6 Mbps at a higher hierarchical level.
As shown in FIG. 3, the multiplex signal transmitted to the radio base station 101 is composed of 32 time slots TS0 .about.TS31. Each of the time slots TS0 .about.TS31, for example, the time slot TS1, comprises auxiliary time slots TS0(R) .about.TS3(R) for accommodating audio/data signals. In this conventional mobile radio communication system, however, the time slot TS3(R) is not used to accommodate a coded audio signal, but to accommodate a monitor/control signal for the CODECs. The control signal processor 11 sends a control signal for connecting calls to and from the mobile stations through switch circuit 7 and a signal line for the call connecting signal a0 to multiplexer/demultiplexer 6. The control signal is then multiplexed and allotted to a certain time slot of the multiplex signal of 2 Mbps by multiplexer/demultiplexer 6, and transmitted to the radio base station 101. The control unit 13 sends a switching control signal to switch circuit 7 and also controls the control signal processor 11 for controlling transmission and reception of control signals to and from the exchange apparatus 103 or the mobile stations.
The radio base station 101 comprises a multiplexer/demultiplexer 5, transmitter/receivers 3-1 .about.3-n, a control signal processor 4, a combiner/distributor 2 for combining signals to be transmitted from the transmitter/receivers 3-1 .about.3-n and distributing received signals to the transmitter/receivers 3-1 .about.3-n. The multiplexer/demultiplexer 5 demultiplexes the multiplex signal from the base station controller 102 into signals a1, a2, . . . , an, and supplies the signals a1, a2, . . . , an to the respective transmitter/receivers 3-1 .about.3-n. The transmitter/receivers 3-1 .about.3-n process the supplied signals a1, a2, . . . , an, using as one group the auxiliary time slots TS0(R) .about.TS2(R) contained in a given time slot of the multiplex signal of 2 Mbps, for transmission over radio communication paths. More specifically, as shown in FIG. 4 of the accompanying drawings, the transmitter/receivers 3-1 .about.3-n process the signal in each of the auxiliary time slots TS0(R) .about.TS2(R) by adding an error correcting code to the signal for recovery from bit errors which may be introduced during transmission over the radio communication paths and interleaving the signal, and add a start word (SW) signal indicative of an identification of the time slot over the radio circuits to the signal. The control signal a0 for connecting a call from the base station controller 102 is demultiplexed by multiplexer/demultiplexer 5, received by the control signal processor 4, and then transmitted in a certain time slot from a certain transmitter/receiver to a mobile station.
The conventional base station controller 102 has CODECs in one-to-one correspondence to the time slots outputted from the respective transmitter/receivers over the radio communication paths. In a system in which three time slots (see FIG. 4) are time-division-multiplexed over the radio circuits, a signal of 64 Kbps composed of time-division-multiplexed output signals from three CODECs is supplied to the transmitter/receivers. Since the CODEC corresponding to the time slot used for controlling the connection of a call over a radio communication path is in use, that CODEC is idling and not effectively utilized.